The overloading of IR emits magic \X'...' sequences that turn into HTML manual links.
But not all such IR invocations should be manual links;
those had to be written to avoid the IR macro before.
Worse, the \X'...' ending the IR causes troff to emit only a single space after a period.
Defining a new IM macro for manual references fixes both problems.
Fixes#441.
This is an experiment.
Like tpic it's a copy-and-paste fork of pic.
Change-Id: Ia22772bd5881c7904a6d8f8e0b46fde8cea89cbd
Reviewed-on: https://plan9port-review.googlesource.com/2920
Reviewed-by: Russ Cox <rsc@swtch.com>
